The regulation of development and cellular differentiation is important for all multicellular organisms. The nitrogen-fixing filamentous cyanobacterium Anabaena (also Nostoc) sp. PCC 7120 (hereafter Anabaena) provides an important model of multicellular microbial development and pattern formation. Anabaena reduces N2 to ammonia in specialized terminally-differentiated cells called heterocysts. A one-dimensional developmental pattern of single heterocysts regularly spaced along filaments of photosynthetic vegetative cells is established to form a multicellular organism composed of these two interdependent cell types. This multicellular growth pattern, the distinct phylogeny of cyanobacteria, and the suspected antiquity of heterocyst development make this an important model system. Our long-term goal is to understand the regulatory network required for heterocyst development. This project is focused on cell-cell signaling and related regulatory pathways that control the initiation of development and pattern formation. An important advance was our identification of patS, which is required for normal pattern formation. Our results indicate that patS encodes a diffusible peptide inhibitor that regulates heterocyst pattern by lateral inhibition. To understand the mechanisms underlying the regulation, we will first identify the components of the PatS signaling pathway that are required for: controlling expression of the patS gene, mediating cell-to-cell communication, receiving the signal in target cells, and producing downstream responses. We will use several approaches to identify genes and proteins required for this regulatory network. The completed Anabaena genome sequence facilitates the analysis of mutants and allows the application of genomics-based techniques such as reverse genetics and expression profiling for defining a molecular phenotype of mutants. Determining the mechanism of PatS signaling will show how groups of apparently equivalent cells can be resolved to a single cell that becomes committed to differentiate into a heterocyst. Our specific objectives are to answer the following five questions. What genes and proteins are required for the production of the PatS signal, its propagation between cells along the filament, and its perception by target cells? What is the mechanism by which patS transcription is temporally and spatially regulated? What proteins interact with PatS? What is the final target of the PatS signaling pathway that controls the decision to differentiate? Do the temporal and spatial expression patterns of genes involved in PatS signaling support a lateral inhibition model for the mechanism controlling pattern formation?